1. Field of the Invention
The present invention relates to a driver circuit for a semiconductor laser diode (hereafter denoted as LD), in particular, the invention relates to a driver circuit for the LD operated in the differential mode.
2. Related Prior Art
Various prior patent applications have disclosed a driver circuit for the LD in which the driver circuit applies a modulation voltage or current directly to the LD, which is called as the direct modulation. One type of the directly modulation circuits provides a switching transistor connected in series to the LD receives a modulation signal in the control node thereof to switch the current applied to the LD directly. The driver circuit often includes an automatic power control (hereafter denoted as APC) circuit that may keep the average power and the extinction ratio of the light output from the LD constant.
Because the LD has large temperature dependence in the emission efficiency thereof, a larger driving current becomes necessary in high temperatures compared with that in a low temperature or in a room temperature to maintain the average power and the extinction ratio. Moreover, the emission efficiency is widely scatted in individual LDs, which requests a driver circuit with a higher power supply to operate an LD with lower emission efficiency even when an LD shows better efficiency.
A Japanese patent application published as JP-H06-275895A and another application published as JP-2002-158395A have disclosed a driver circuit implemented with a voltage converter for the power supply of the driver circuit. The former application, JP-H06-275895A, provides a circuit to monitor a bias condition between the drain and the source of the transistor, and adjusts the bias applied to the LD based on this monitored condition. While, the latter application, JP-2002-158395A, has a circuit to monitor the drain bias of the transistor and adjusts the bias to the LD by this monitored drain bias.
One type of driving modes for the LD has been known as the differential mode, in which the driver circuit provides a paired transistors each having a load resistor to constitute a differential circuit, and the LD is connected between the outputs of the outputs of the differential circuit to modulate the light output from the LD. Moreover, even when the driver circuit is configured with the differential mode, the APC circuit may adjust the modulation current and the bias current to keep the average power and the extinction ratio of the light output from the LD constant.
Eve in such a driver circuit with the differential mode, it is necessary for the driver circuit to provide a power supply with higher voltages because the LD is necessary to be supplied with a larger current due to a less emission efficient at high temperatures, which becomes a barrier for the lower power consumption. However, the driver circuit with the differential mode is inevitable to secure the complementarity of the paired transistor, which restricts the prior technique disclosed in the application above listed to be implemented within the driver circuit with the differential mode.
Therefore, the present invention is to provide a driver circuit for the LD with the differential mode that is able to reduce the power consumption.